recirculation usually turns on MAX A/C which on some cars will leave the compressor running continuously. It will seem to blow harder at slow speeds but on some vehicles it will blow harder when you go fast when set to not recirculate because the air is being forced in (noticed this mostly on my truck)

normally I leave my A/C on my Focus on MAX just because once the air is cool, it doesn't have to re-cool outside air that is hot